Dual Thurston polytope
= Dual Thurston polytope
{title2=$B_T(Y)$}
The dual polytope consists of the real linear functionals $\alpha$ on $H_2(Y,\partial Y;\mathbb R)$ such that $|\alpha(u)|\leq\|u\|_T$ for every $u$. Thus it is the polar of the unit ball of the <Thurston norm> and annihilates that <seminorm>'s kernel.